1. Field of the Invention
The present invention relates to an imaging apparatus, and, more particularly, to an ink jet printer utilizing an improved dot placement technique.
2. Description of the Related Art
A typical ink jet printer forms an image on a print medium by ejecting ink from at least one ink jet printhead to form a pattern of ink dots on the print medium. Such an ink jet printer includes a reciprocating printhead carrier that transports one or more ink jet printheads across the print medium along a bi-directional scanning path defining a print zone of the printer. The bi-directional scanning path is oriented parallel to a main scan direction, also commonly referred to as the horizontal direction. The main scan direction is bi-directional. During each scan of the printhead carrier, the print medium is held stationary. An indexing mechanism is used to incrementally advance the print medium in a sheet feed direction, also commonly referred to as a sub-scan direction or vertical direction, through the print zone between scans in the main scan direction, or after all data intended to be printed with the print medium at a particular stationary position has been completed.
For a given stationary position of the print medium, printing may take place during one or more unidirectional scans of the printhead carrier. As used herein, the term xe2x80x9cunidirectionalxe2x80x9d will be used to refer to scanning in either, but only one, of the two bi-directional scanning directions. Thus, bi-directional scanning refers to two successive unidirectional scans in opposite directions. The term xe2x80x9cprinting swathxe2x80x9d will refer to the depositing of ink on the print medium during a particular unidirectional scan of the printhead carrier at which time individual printhead nozzles of the printhead are selectively actuated to expel ink. A printing swath is made of a plurality of printing lines traced along imaginary rasters, the imaginary rasters being spaced apart in the sheet feed direction.
Typically, each ink jet printhead will include a plurality of ink jet nozzles for expelling the ink. In ink jet printing, it is common to use the ink colors of cyan, magenta, yellow and black in generating color prints. Also, it is common in ink jet printing to have a single printhead having a dedicated nozzle array for each of cyan, magenta and yellow inks, respectively, wherein the three nozzle arrays are aligned vertically, that is, aligned in a direction parallel to the sub-scan direction.
Those working in the imaging arts continually strive to improve the print quality of imaging devices, such as ink jet printers.
One such attempt is directed to reducing the occurrence of horizontal banding defects in printouts generated by an ink jet printer. Horizontal banding defects may be observed on media, such as paper, as a horizontal white band. Such defects are generally attributable to errors generated by the media sheet indexing mechanism that is used to advance a media sheet in a media feed direction through the printer during the printing of the text or image on the media sheet. Such errors can be caused, for example, by mechanical tolerances of the index roller and its associated drive train. It is known to mask such indexing errors by adopting an interlaced printing method, also referred to as shingling, wherein each scan of the printhead carrier (also sometimes referred to in the art as a printhead carriage) is made to vertically overlap a preceding scan. For a given swath, only a portion of the total print data for a given area on the print medium is printed. Thus, each scan of an actuated printhead produces a swath of printed output forming all or portions of multiple print lines, and multiple swaths may be required to complete the printing of any given print line.
Other attempts have been made to improve the print quality of high density printed images by reducing the occurrences of ink pen starvation, ink droplet trajectory errors and fuzzy text edges. For example, in one such attempt, an inkjet printer includes a controller and algorithm for switching automatically intra page between one of two independent high speed carriage velocities and between one of two independent pen firing frequencies based on ink drop densities, wherein when the drop density increases to a maximum level, the printer reduces its carriage velocity and nozzle firing rate to allow sufficient time for the ink deposited onto the media to dry.
Another type of printing defect has been recognized, referred to herein as vertical banding. Vertical banding defects in multi-color printing are typically observed as a repeating pattern of vertical light bands and vertical dark bands in a printed image, and may also appear in multi-color form similar to that of a rainbow. Vertical banding defects are particularly noticeable in high density ink jet printer printouts, such as when attempting to produce photographic quality printouts, but also can be observed in lower density printouts as well.
What is needed in the art is a method to reduce vertical banding in an imaging apparatus, such as an ink jet printer, and hence improve the print quality thereof.
The present invention provides a method to reduce vertical banding in an imaging apparatus, such as an ink jet printer, and hence improve the print quality thereof.
The invention, in one form thereof, relates to a method to reduce vertical banding defects in an ink jet printer having a printhead carrier system including a printhead carrier and a carrier motor. The method includes the steps of identifying a disturbance frequency of a disturbance source; identifying a natural frequency of a printhead carrier system; correlating the disturbance frequency of the disturbance source and the natural frequency of the printhead carrier system to a base carrier velocity; selecting a first carrier velocity for printing in a first direction, the first carrier velocity being different from the base carrier velocity; and selecting a second carrier velocity for printing in a second direction, the second carrier velocity being different from the base carrier velocity and different from the first carrier velocity.
In another form thereof, the invention relates to an ink jet printer. The ink jet printer includes a printhead. A printhead carrier system is provided including a printhead carrier for carrying the printhead, the printhead carrier being transported by a carrier motor in a bi-directional scanning path in a reciprocating manner in a first direction and a second direction, the second direction being opposite to the first direction. A controller is communicatively coupled to the printhead and to the printhead carrier system. The controller executes instructions to perform the steps of storing a plurality of carrier velocities in the controller; selecting a first carrier velocity from the plurality of carrier velocities for printing in the first direction; and selecting a second carrier velocity from the plurality of carrier velocities for printing in the second direction, the second carrier velocity being different from the first carrier velocity, wherein the plurality of carrier velocities are selected to reduce vertical banding defects in the ink jet printer.
In still another form, the invention relates to a method of printing in a printer having a printhead carrier, the printhead carrier being driven to scan in a first direction and a second direction opposite to the first direction. The method includes the steps of identifying a base carrier velocity; selecting a first carrier velocity; selecting a second carrier velocity, different from the first carrier velocity, wherein an average of the first carrier velocity and the second carrier velocity is substantially equal to the base carrier velocity; selecting one of the first carrier velocity and the second carrier velocity for scanning the printhead carrier in the first direction; and selecting the other of the first carrier velocity and the second carrier velocity for scanning the printhead carrier in the second scan direction.